Sign a certain captain has turned up (4)
I believe the answer is:
omen
'sign' is the definition.
(omen is a kind of sign)
'a certain captain has turned up' is the wordplay.
'a certain captain' becomes 'nemo' (Captain Nemo, Jules Verne character).
'has turned up' shows that the letters should be reversed in order.
'nemo' backwards is 'OMEN'.
(Other definitions for omen that I've seen before include "Augur" , "Portent of evil" , "Sign of likely future event" , "Auspice" , "Portent, prophetic sign" .)